Image Guided Surgery (IGS), also known as “frameless stereotaxy” has been used for many years to precisely locate and position therapeutic or medical measurement devices in the human body. Proper localization including position and orientation of these devices is critical to obtain the best result and patient outcome.
Some image guided surgery techniques use an externally placed locating device, such as a camera system or magnetic field generator together with an instrument containing a trackable component or “position indicating element” that can be localized by a locating device or tracking system (collectively referred to hereinafter a “tracking device”). These position indicating elements are associated with a coordinate system and are typically attached to instruments such as surgical probes, drills, microscopes, needles, X-ray machines, etc. and to the patient. The spatial coordinates and often the orientation (depending on the technology used) of the coordinate system associated with the position indicating elements can be determined by the tracking device in the fixed coordinate system (or fixed “frame of reference”) of the tracking device. Many tracking devices may be able to track multiple position indicating elements simultaneously in their fixed frame of reference. Through geometrical transformations, it is possible to determine the position and orientation of any position indicating element relative to a frame of reference of any other position indicating element.
A variety of different tracking devices exist, having different advantages and disadvantages over each other. For example, optical tracking devices may be constructed to enable the highly accurate position and orientation of a tool equipped with position indicating elements to be calculated. However, these optical tracking devices suffer from line-of-site constraints, among other things. Electromagnetic (EM) tracking devices do not require a line-of-sight between the tracking device and the position indicating elements. Electromagnetic tracking devices may therefore be used with flexible instruments where the position indicating elements are placed at the tip of the instruments. One disadvantage, however, is that electromagnetic tracking devices are subject to interference from ferromagnetic materials and conductors. This interference may degrade accuracy when such ferromagnetic materials or conductors are placed in the proximity of position indicating elements or EM tracking devices. Other known tracking devices include, but are not limited to, fiber optic devices, ultrasonic devices and global positioning (“time of flight”) devices.
By combining data obtained from a tracking device and a position indicating element with preoperative or intraoperative scans (such as for example, x-rays, ultrasounds, fluoroscopy, computerized tomographic (CT) scans, multislice CT scans, magnetic resonance imaging (MRI) scanning, positron emission tomographic (PET) scans, isocentric fluoroscope images, rotational fluoroscopic reconstructions, intravascular ultrasound (IVUS) images, single photon emission computer tomographer (SPECT) systems, or other images), it is possible to graphically superimpose the location of the position indicating element (and thus any surgical instrument having a position indicating element) over the images. This enables the surgeon to perform an intervention/procedure more accurately since the surgeon is better able to locate or orient the instrument during the procedure. It also enables the surgeon to perform all or part of the procedure without the need for additional x-rays or other images, but instead to rely on previously acquired data. This not only reduces the amount of ionizing radiation the surgeon and patient are exposed to, but can speed the procedure and enable the use of higher fidelity images than can not normally be acquired intra-operatively. Surgical plans may also be annotated onto these images (or indeed used without the images) to be used as templates to guide medical procedures.
Image Guided Surgery can be most effectively performed only if an accurate “registration” is available to mathematically map the position data of position indicating elements expressed in terms of the coordinate system of the tracking device, i.e., “patient space,” to the coordinate system of the externally imaged data, i.e., “image space” determined at the time the images were taken. In rigid objects such as the skull or bones, one method of registration is performed by using a probe equipped with position indicating elements (therefore, the probe itself is tracked by a tracking device) to touch fiducial markers (such as, for example, small steel balls (x-spots) made by the Beekley Corporation, Bristol, Conn.) placed on the patient to obtain the patient space coordinates of the fiducials. These same fiducials are visible on an image such as, for example, a CT scan and are identified in the image space by indicating them, for example, on a computer display. Once these same markers are identified in both spaces, a registration transformation or equivalent mathematical construction can be calculated. In one commonly used form, a registration transformation may be a 4.times.4 matrix that embodies the translations, magnification factors and rotations required to bring the markers (and thus the coordinate systems) in one space in to coincidence with the same markers in the another space.
Fiducial markers used for registration can be applied to objects such as bone screws or stick-on markers that are visible to the selected imaging device, or can be implicit, such as unambiguous parts of the patient anatomy. These anatomical fiducials might include unusually shaped bones, osteophytes or other bony prominence, features on vessels or other natural lumens (such as bifurcations), individual sulci of the brain, or other markers that can be unambiguously identified in the image and patient. A rigid affine transformation such as the 4.times.4 matrix described above may require the identification of at least three non-collinear points in the image space and the patient space. Often, many more points are used and a best-fit may be used to optimize the registration. It is normally desirable that fiducials remain fixed relative to the anatomy from the time of imaging until the time that registration is complete.
Registration for image-guided surgery may be done by different methods. Paired-point registration is described above and is accomplished by a user identifying points in image space and then obtaining the coordinates of the corresponding points in patient space. Another type of registration, surface registration, can be done in combination with, or independent of, paired point registration. In surface registration, a cloud of points is digitized in the patient space and matched with a surface model of the same region in image space. A best-fit transformation relating one surface to the other may then be calculated. In another type of registration, repeat-fixation devices may be used that involve a user repeatedly removing and replacing a device in known relation to the patient or image fiducials of the patient.
Automatic registration may also be done. Automatic registration may, for example, make use of predefined fiducial arrays or “fiducial shapes” that are readily identifiable in image space by a computer. The patient space position and orientation of these arrays may be inferred through the use of a position indicating element fixed to the fiducial array. Other registration methods also exist, including methods that attempt to register non-rigid objects generally through image processing means.
Registrations may also be performed to calculate transformations between separately acquired images. This may be done by identifying “mutual information” (e.g., the same fiducial markers existing in each space). In this way, information visible in one image, but not the other, may be coalesced into a combined image containing information from both. In the same manner, two different tracking devices may be registered together to extend the range of a tracking device or to increase its accuracy.
Following registration, the two spaces (patient and image) are linked through the transformation calculations. Once registered, the position and orientation of a tracked probe placed anywhere in the registered region can be related to, for example, a scan of the region. Typically the tracking device may be connected to a computer system. Scans may also be loaded onto the computer system. The computer system display may take the form of a graphical representation of a probe or instrument's position superimposed onto preoperative image data. Accordingly, it is possible to obtain information about the object being probed as well as the instrument's position and orientation relative to the object that is not immediately visible to the surgeon. The information displayed can also be accurately and quantitatively measured enabling the surgeon to carry out a preoperative plan more accurately.
An additional concept in image guided surgery is that of “dynamic referencing.” Dynamic referencing can account for any bulk motion of the anatomy relative to the tracking device. This may entail additional, position indicating elements, or other techniques. For example, in cranial surgery, position indicating elements that form the dynamic reference are often attached directly to the head or more typically to a clamp meant to immobilize the head. In spine surgery, for example, a dynamic reference attached (via a temporary clamp or screw) to the vertebral body undergoing therapy is used to account for respiratory motion, iatrogentic (e.g., doctor-induced) motion caused by the procedure itself, as well as motion of the tracking device. In an analogous manner, the tracking device itself may be attached directly to the anatomy, moving with the anatomy when it moves. For example, a small camera may be attached to a head-clamp so that movement of the head would produce movement of the camera, thus preserving registration.
“Gating” may also be used to account for motion of the anatomy. Instead of continually compensating for motion through dynamic referencing, “gated measurements” are measurements that are only accepted at particular instants in time. Gating has been used in, for example, cardiac motion studies. Gating synchronizes a measured movement (e.g., heartbeat, respiration, or other motion) to the start of the measurement in order to eliminate the motion. Measurements are only accepted at specific instants. For example, gating during image guided surgery of the spine may mean that the position of a tracked instrument may be sampled briefly only during peak inspiration times of a respiratory cycle.
Both registration and use of an image guided surgery system in the presence of anatomical motion (such as that which occurs during normal respiration) is generally regarded as safer and more accurate if a dynamic reference device is attached prior to registration (and/or if gating is used). Instead of reporting the position and orientation of a position indicating element of a tracked instrument in the fixed coordinate system of the tracking device, the position and orientation of the position indicating element of the tracked instrument is reported relative to the dynamic reference's internal coordinate system. Any motion experienced mutually by both the dynamic reference and the tracked instrument is “cancelled out.”
There are many difficulties and problems in image guided surgery and the prior techniques. These are not limited to, but include: (a) obtaining adequate registration and (b) adequately dynamically referencing the anatomy or a portion thereof, and (c) verifying that registration is accurate enough to perform the procedure using image guidance.
Paired-point registration in rigid or near-rigid anatomical objects can be accomplished using direct probing. Paired-point registration is less-attractive when the anatomical object is either inaccessible, non-rigid, or both. When the anatomical object is not accessible but rigid (such as pelvic bone), it may be necessary to either palpate the surface of the object through probing through an opening in the skin, or through the rigid attachment of a palpatable registration object prior to imaging. In rigid and non-rigid organs or anatomically connected regions, methods such as ultrasound and laser surface scanning are used with varying amounts of success.
Registration and referencing of non-rigid and/or moving organs such as the liver, gall bladder, stomach, pancreas, kidney, lung, colon, heart, prostate gland, etc. is a difficult task. Use of devices such as probes generally deform the organ. Furthermore, it is difficult to attach any kind of dynamic reference to a soft moving object. Such organs tend to be generally inaccessible directly through the skin without damaging intervening tissue or the organ itself. Techniques using ultrasound are complicated by different sound velocities and attenuation from different tissues.
Current registration and dynamic referencing techniques usually assume that the tracked organ is rigid. Newer techniques are being proposed that are not limited to rigid organs. These can benefit from placement of multiple dynamic references. However, problems still exist since multiple dynamic references must be temporarily fixed to a deformable anatomical object.
Another limitation of current dynamic referencing techniques stems from the use of reference sensors. The most widely used method of referencing known in the art is to place a single, rigid, six degree-of-freedom (6 DOF), trackable device onto the organ of interest. This is typically screwed, clamped or otherwise rigidly attached to the organ of interest. Such rigid bodies are typically of large footprint, and give information only at the location to which they are attached. In the event that they are attached some distance from the site of intervention/procedure, or attached to a non-rigid object, the motion of the dynamic reference may not accurately track the motion at the site of intervention/procedure.
Another limitation of current image guided surgery techniques may include the difficulty of verifying that a registration has been performed correctly. Before proceeding to navigate the anatomical region of the patient based on preoperative images and registration, it may be important to ensure that the registration is accurate. In image guided procedures in hard tissue, a probe can be touched onto hard surfaces or features (after registration) to ensure the registration is accurate. This technique suffers from the same issues as registration itself in soft tissue, e.g., deformation of soft tissue during verification, access to the tissue, paucity of verification landmarks, and other problems.
A further limitation of current approaches is the amount of fluoroscopy that must be used to correctly position the therapy device in the event that image guided surgery is not used. While it does provide accurate and direct information of the progress of the intervention, images are two-dimensional in nature and require continuous exposure of the patient and surgical team to ionizing radiation. Three-dimensional images may be more useful.
These and other problems exist.